A. Field of the Invention
The present invention relates to a method of sequence-specific recombination of DNA in eukaryotic cells, comprising the introduction of a first DNA comprising a nucleotide sequence containing at least one recombination sequence into a cell, introducing a second DNA comprising a nucleotide sequence containing at least one further recombination sequence into a cell, and performing the sequence specific recombination by a bacteriophage lambda integrase Int.
B. Related Art
The controlled manipulation of eukaryotic genomes and the expression of recombinant proteins from episomal vectors are important methods for analyzing the function(s) of specific genes in living organisms. Moreover, said manipulations play a role in gene therapeutic methods in medicine. In this context the generation of transgenic animals, the change of genes or gene segments (so-called “gene targeting”) and the targeted integration for foreign DNA into the genome of higher eukaryotes are of particular importance. Recently, these technologies could be improved by means of characterization and application of sequence specific recombination systems.
Furthermore, sequence-specific integration of expression cassettes, encoding and expressing a desired polypeptide/product, into the genome of biotechnological relevant host cells also gets more significance for the production of biopharmaceuticals. Expression level for a desired polypeptide in a stable transformed cell lines depends on the site of integration. By sequence specific integration, sites could be preferably used having a high transcription activity. The conventional method for generating production cell lines expressing a desired polypeptide/product is based on the random integration of the recombinant expression vector into the genome of the host cell. Variations in the expression level of the integrated gene(s) of interest in stable transformed cell lines are attributed mainly to differences in chromosomal locations and copy numbers. Random integration in the proximity of heterochromatin results in variable levels of transgene expression. Chromosome locations promoting the expression of the integrated gene(s) of interest are thought to be transcriptionally active regions of euchromatin. This randomness of integration causes a large diversity in recombinant cells robustness, productivity and quality, necessitating an elaborate screening process to identify and isolate a suitable cell clone producing the desired polypeptide at high level. In addition, the heterogeneity also means that for each clone an optimized production process has to be developed, making the development of a suitable production cell line a time consuming, labor intensive and costly process.
Conservative sequence specific DNA recombinases have been divided into two families. Members of the first family, the so-called “integrase” family, catalyze the cleavage and rejoining of DNA strands between two defined nucleotide sequences, which will be named as recombination sequences in the following. The recombination sequences may be either on two different or on one DNA molecule, resulting in inter- or intramolecular recombination, respectively. For intramolecular recombination, the result of the reaction depends on the respective orientation of the recombination sequences to each other. In the case of an inverted, i.e., opposite orientation of the recombination sequences, inversion of the DNA segments lying between the recombination sequences occurs. In the case of direct, i.e., tandem repeats of the recombination sequences on a DNA substrate, a deletion occurs. In case of the intermolecular recombination, i.e., if both recombination sequences are located on two different DNA molecules, a fusion of the two DNA molecules may occur. While members of the integrase family usually catalyze both intra- as well as intermolecular recombination, the recombinases of the second family of the so-called “invertases/resolvases” are only able to catalyze the intramolecular recombination.
At present, the recombinases which are used for the manipulation of eukaryotic genomes belong to the integrase family. Said recombinases are the Cre recombinase of the bacteriophage P1 and the Flp recombinase from yeast (Müller, 1999). The recombination sequences to which the Cre recombinase binds are named loxP. LoxP is a 34 bp long nucleotide sequence consisting of two 13 bp long inverted nucleotide sequences and an 8 bp long spacer lying between the inverted sequences (Hoess et al., 1985). The FRT named binding sequences for Flp are build up similarly. However, they differ from loxP (Kilby et al. 1993. Therefore, the recombination sequences may not be replaced by each other, i.e., Cre is not able to recombine FRT sequences and FLP is not able to recombine loxP sequences. Both recombination systems are active over long distances, i.e., the DNA segment to be inverted or deleted and flanked by two loxP or FRT sequences may be several 10 000 base pairs long.
For example, a tissue specific recombination in a mouse system, a chromosomal translocation in plants and animals, and a controlled induction of the gene expression was achieved with said two systems; review article of Müller, (1999). The DNA polymerase β was deleted in particular tissues of mice in this way; Gu et al. (1994). A further example is the specific activation of the DNA tumor virus SV40 oncogene in the mouse lenses leading to tumor formation exclusively in these tissues. The Cre-loxP strategy was used also in connection with inducible promotors. For example, the expression of the recombinase was regulated with an interferon-inducible promotor leading to the deletion of a specific gene in the liver and not—or only to a low extent—in other tissues; Kühn et al. (1995).
So far, three members of the invertase/resolvase family have been used for the manipulation of eukaryotic genomes. A mutant of the bacteriophage Mu invertase Gin can catalyze the inversion of a DNA fragment in plant protoplasts without cofactors. However, it has been discovered that this mutant is hyper-recombinogenic, i.e., it catalyzes DNA strand cleavages also at other than its naturally recombination sequences. This leads to unintended partially lethal recombination events in plant protoplast genomes. The β-recombinase from Streptococcus pyogenes catalyses the recombination in mouse cell cultures between two recombination sequences as direct repeats leading to the excision of the segment. However, simultaneously with deletion also inversion has been detected which renders the controlled use of the system for manipulation of eukaryotic genomes unsuitable. Mutants of the γδ resolvase from E. coli have been shown to be active on episomal and artificially introduced genomic recombination sequences, but the efficiency of the latter reaction is still rather poor.
The manipulation of eukaryotic genomes with the Cre and Flp recombinase, respectively, shows significant disadvantages. In case of deletion, i.e., the recombination of two tandem repeated loxP or FRT recombination sequences in a genome there is an irreversibly loss of the DNA segment lying between the tandem repeats. Thus, a gene located on this DNA segment will be lost permanently for the cell and the organism. Therefore, the reconstruction of the original state for a new analyses of the gene function, e.g., in a later developmental stage of the organism, is impossible. The irreversible loss of the DNA segment caused by deletion may be avoided by an inversion of the respective DNA segment. A gene may be inactivated by an inversion without being lost and may be switched on again at a later developmental stage or in the adult animal by means of a timely regulated expression of the recombinase via back recombination. However, the use of both Cre and Flp recombinases in this modified method has the disadvantage that the inversion cannot be regulated as the recombination sequences will not be altered as a result of the recombination event. Thus, repeated recombination events occur causing the inactivation of the respective gene due to the inversion of the respective DNA segment only in some, at best in 50% of the target cells at equilibrium of the reaction. There have been efforts to solve this problem, at least in part, by constructing mutated loxP sequences which cannot be used for further reaction after a single recombination. However, the disadvantage is the uniqueness of the reaction, i.e., there is no subsequent activation by back recombination after inactivation of the gene by inversion.
A further disadvantage of the Flp recombinase is its reduced heat stability at 37° C. thus limiting the efficiency of the recombination reaction in higher eukaryotes significantly, e.g., in mice with a body temperature of about 39° C. Therefore, Flp mutants have been generated which exhibit a higher heat stability as the wild-type recombinase. However, even these mutant Flp enzymes still exhibit a lower recombination efficiency than the Cre recombinase.
A further use of sequence specific recombinases resides in the medical field, e.g., in gene therapy, where the recombinases integrate a desired DNA segment into the genome of a respective human target cell in a stable and controlled way. Both Cre and Flp may catalyze intermolecular recombination. Both recombinases recombine a plasmid DNA which carries a copy of its respective recombination sequence with a corresponding recombination sequence which has been inserted before into the eukaryotic genome via homologous recombination. However, it is desirable that this reaction includes a “naturally” occurring recombination sequence in the eukaryotic genome. Because loxP and FRT are 34 and 54 nucleotides long, respectively, occurrence of exact matches of these recombination sequences as part of the genome is statistically unlikely. Even if a recombination sequence would be present, the disadvantage of the aforementioned back reaction still exists, i.e., both Cre and Flp recombinase may excise the inserted DNA segment after successful integration by intramolecular recombination.
Thus, one problem of the present invention is to provide a simple and controllable recombination system, and the required working means. A further problem of the present invention is the provision of a recombination system and the required working means, which may carry out a stable and targeted integration of a desired DNA sequence. A further problem of the present invention is the provision of methods which allows the generation of an improved protein expression system on the basis of one of those recombination systems.